I need to design an arrangment of solid-state gates such that a pump will start filling a tank only if it is MT (not part full) & stop when it is full.

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In terms of what you have described, the tank can have only 3 states of condition. Either its full, empty or part full. I understand the pump is to stop when the tank is full but what is meant by "not part full"? Does that mean empty?

Originally posted by David Bridgen@May 16 2005, 04:39 AM....... and which column corresponds to which sensor?

[post=7712]Quoted post[/post]​

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column one is for the sensor at the botom of the tank & column two for the one at the top.

I can work out how to draw the circuit for it, the problem I am having is that either the pump starts when it is MT & stops when it is part full (using an AND gate) or the pump starts when it is MT or part full and stops when the tank is full (using an OR gate).

The bit I don't understand is how I am meant to make it only start when the tank is MT and only stop when it is full.

Originally posted by gsd@May 16 2005, 04:24 AMcolumn one is for the sensor at the botom of the tank & column two for the one at the top.

I can work out how to draw the circuit for it, the problem I am having is that either the pump starts when it is MT & stops when it is part full (using an AND gate) or the pump starts when it is MT or part full and stops when the tank is full (using an OR gate).

The bit I don't understand is how I am meant to make it only start when the tank is MT and only stop when it is full.

[post=7720]Quoted post[/post]​

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I know one picture is worth a thousand words, but time is pressing so here's just a description.

Try this. Use three 2input NOR gates - A, B, and C.
Connect A and B to form a bistable by connecting A o/p to B i/p, and B o/p to A i/p. When the two spare inputs are low. the circuit will be a bistable. This circuit works on the basis that when an input is pulsed high, the pulse will flip the bistable to turn on that gate, and it will stay that way when the pulse is removed.

Connect the bottom switch (BS) to the spare A i/p. Connect the top switch (TS) to the two C inputs tied together. C now acts as an invertor. Connect its o/p to the spare B input. Use the B output for the motor drive (high = motor on).

Let's assume A o/p is high and B is low (motor off) and tank is draining. TS is high so C o/p is low. When empty, BS switches high, turning on A and turning off B (o/p high). Motor runs, filling the tank. BS changes to low, but that will not affect the bistable.

When tank is full, TS goes low (C o/p high) turning on B and turning off A. Motor stops. As tank empties, TS goes high but doesn't change bistab. The cycle repeats when the tank is empty.